The present invention relates to a method for driving an ink jet recording head which method ejects fine ink droplets through a nozzle to record characters or images.
One of such recording heads, what is called an on-demand ink jet recording head that ejects ink droplets through a nozzle depending on printed information, is conventionally commonly known (for example, see Japanese Patent Publication No. SHO 53-12138). FIG. 15 is a sectional view schematically showing a basic configuration of one of such on-demand ink jet recording heads which is called a Kyser type.
In this Kyser type recording head, on an ink upstream side, a pressure generating chamber 91 and a common ink chamber 92 are connected together via an ink supply hole (ink supply passage) 93, and on an ink downstream side, the pressure generating chamber 91 and a nozzle 94 are connected together, as shown in FIG. 15. Additionally, a bottom plate portion of the pressure generating section 91, which is located at the bottom of FIG. 15, comprises a diaphragm 95 having a piezoelectric actuator 96 on its rear surface.
With this configuration, during a printing operation, the piezoelectric actuator 96 is driven depending on printed information to displace the diaphragm 95, thereby changing the volume of the pressure generating chamber 91 rapidly to generate a pressure wave in the pressure generating section 91. The pressure wave causes a part of an ink filled in the pressure generating chamber 91 to be injected to an exterior through the nozzle 94 and ejected as ink droplets 97. The ejected ink droplets 98 arrived in a recording medium such as recording paper to form recording dots. Characters or images are recorded on the recording medium by repeating the formation of recording dots based on printing information.
The ink droplet ejecting operation will be further described. With this on-demand ink jet recording method or system, a single ink droplet is ejected whenever a driving voltage is applied to the piezoelectric actuator 96. In the prior art, however, to eject a single ink droplet, a trapezoidal driving voltage waveform is generally applied to the piezoelectric actuator 96.
The trapezoidal driving voltage waveform comprises a first voltage changing process 51 for linearly increasing a voltage V applied to the piezoelectric actuator 96 from a reference value up to a predetermined value V1 to compress the pressure generating chamber 91 to eject the ink droplet 97, a voltage maintaining process 52 for maintaining the applied voltage V at the predetermined value V1 for a certain amount of time (time t1xe2x80x2), and a second voltage changing process 53 for subsequently returning the applied voltage V1 to the reference voltage to return the compressed pressure generating chamber 91 to its original state, as shown in FIG. 16.
Movement of the piezoelectric actuator caused by an increase or decrease in driving voltage depends on the structure or polarization of the piezoelectric actuator, so some piezoelectric actuators move in a direction opposite to the movement direction of the above-mentioned piezoelectric actuator. Since, however, the reversely operating piezoelectric actuator performs an ejection operation similar to that described above when an opposite driving voltage is applied, a piezoelectric actuator that moves in a direction that compresses the pressure generating chamber when the applied voltage increases, while moving in a direction that inflates the pressure generating chamber when the applied voltage decreases will be described in the following xe2x80x9cBEST MODE FOR CARRYING OUT THE INVENTIONxe2x80x9d for simple explanation.
In this ink jet recording head, since a single pixel is formed when the ink droplet 97 impacts on recording paper to form a recording dot, if the recording dot has a large diameter, it appears granular to prevent high image quality from being obtained. Thus, a dot size required to obtain a smooth image that does not appear granular (high image quality) is empirically assumed to be 40 xcexcm or less, and a dot size of 25 xcexcm or less is considered very preferable. Evidently, the size of the ejected ink droplet 97 may be reduced in order to obtain a small dot size. The relationship between the ink droplet size and the dot size depends on a flying speed (droplet speed) of the ink droplet 97, a physical property of the ink (e.g. viscosity or surface tension), the type of recording paper, or the like, but the dot size is normally about twice as large as the ink droplet size. Consequently, to obtain a dot size of 40 xcexcm, the ink droplet size must be 20 xcexcm, and to obtain a smaller size, for example, a dot size of 25 xcexcm or less, the ink droplet size must be 12.5 xcexcm or less.
On the other hand, it is theoretically known that if the ink droplet 97 is to be ejected through the nozzle 94 using a pressure wave, the volume q of the ejected ink droplet 97 is proportional to {circle around (1)} the opening area An of the nozzle 94, {circle around (2)} the speed (droplet speed) Vd of the ink droplet 97, and {circle around (3)} the resonance frequency (specific cycle) Tc of the pressure wave in the pressure generating chamber 91 (acoustic fundamental vibration mode) in the as shown in Equation (1). Accordingly, to reduce the size of the ink droplet 97, the nozzle opening diameter, the droplet speed Vd, and the resonance frequency Tc of the pressure wave may be correspondingly reduced.
qxe2x88x9dTcVdAnxe2x80x83xe2x80x83(1)
Thus, first, the resonance frequency Tc of the pressure wave will be discussed. The resonance frequency Tc of the pressure wave is reduced by reducing the volume of the pressure generating chamber 91 or increasing the rigidity of walls of the pressure generating chamber while reducing the acoustic capacity of the pressure generating chamber 91. When, however, the resonance frequency Tc of the pressure wave is extremely reduced, for example, down to the order of several xcexcs, a refilling operation is prevented from being operated smoothly, resulting in adverse effects on ejection efficiency, maximum driving frequency, or the like. Accordingly, the resonance frequency Tc of the pressure wave has a minimum limit between 10 and 20 xcexcs.
Next, the droplet speed Vd of the ink droplet 97 will be described. The droplet speed Vd affects the impact position accuracy of the ink droplet 97, and a lower droplet speed reduces the impact position accuracy of the ink droplet 97 because the ink droplet 97 is affected by an air flow. Consequently, the droplet speed Vd of the ink droplet 97 cannot be extremely reduced only to reduce the droplet size, and must after all have a fixed value or more (normally about 4 to 10 m/s) in order to obtain high image quality.
Next, the nozzle opening diameter will be described. Due to the above described reasons, it is empirically known that if the resonance frequency Tc of the pressure wave in the pressure generating chamber 91 filled with an ink is set between about 10 and 20 xcexcs, the droplet speed Vd of the ink droplet 97 is set between about 4 and 10 m/s, and the piezoelectric actuator 96 is driven using the driving voltage waveform shown in FIG. 16, then the minimum ink droplet size obtained is equivalent to the nozzle diameter 97. Accordingly, to obtain an ink droplet size of 20 xcexcm, the nozzle diameter must be 20 xcexcm, and to obtain an ink droplet size less than 20 xcexcm, the nozzle diameter must be less than 20 xcexcm. Forming a nozzle diameter less than 20 xcexcm, however, makes manufacturing very difficult and increases the likelihood that the nozzle is blocked, thus significantly degrading the reliability and durability of the head. Thus, in fact, a nozzle diameter between 25 and 30 xcexcm is presently a lower limit, so that under the above described conditions, the minimum droplet size obtained is between about 25 and 30 xcexcm. It is expected that if the blocking problem is solved in the future, the lower limit of the nozzle diameter will extend to about 20 xcexcm.
As a means for solving these problems, an ink jet driving method has been provided which applies an inversely trapezoidal driving voltage waveform to the piezoelectric actuator 96 to execute xe2x80x9cpull and pushxe2x80x9d to thereby eject ink droplets smaller than the nozzle diameter, as described, for example, in Japanese Patent Laid-Open No. SHO 55-17589.
This driving voltage waveform comprises a first voltage changing process 54 for reducing the voltage V applied to the piezoelectric actuator 96, which is set at a reference voltage V1 ( greater than 0 V), down to, for example, 0 V in order to inflate the pressure generating chamber 91, a voltage maintaining process 55 for maintaining the reduced applied voltage V at 0 V for a certain amount of time (time t1xe2x80x2), and a second voltage changing process 56 for subsequently compressing the pressure generating chamber 91 to eject the ink droplet 97, while increasing the voltage V applied to the piezoelectric actuator 96 up to the original voltage V1 in order to provide for the next ejection, as shown in FIG. 17.
When the pressure generating chamber is thus inflated immediately before the ejection, meniscus present at a nozzle opening surface is drawn to an interior of the nozzle, so that the ejection is started in a state where the meniscus has a depressed shape. Accordingly, this method is called xe2x80x9cmeniscus controlxe2x80x9d, xe2x80x9cpull and pushxe2x80x9d or the like.
According to this xe2x80x9cmeniscus control (pull and push)xe2x80x9d driving method, the meniscus is drawn to the interior of the nozzle immediately before the ejection to reduce the amount of ink inside the nozzle, and ink droplets of a size smaller than the nozzle diameter are formed due to a change in droplet forming conditions before the ejection, thus achieving high quality recording. In addition to this, ejected ink droplets are unlikely to be affected by wetting of the nozzle opening surface, thereby making the ejection more stable.
In addition, Japanese Patent Laid-Open No. SHO 59-143655 proposes a means for using the meniscus control to modulate the droplet size by varying the amount of meniscus receding immediately before the ejection to eject ink droplets of different sizes through the same nozzle.
Further, several proposals have been made for the waveform of the driving voltage used for the meniscus control. For example, Japanese Patent Laid-Open No. SHO 59-218866 defines a time interval (timing) between the first voltage changing process 54 and the second voltage changing process 56 as a condition for easily obtaining fine droplets. Additionally, Japanese Patent Laid-Open No. HEI 2-192947 discloses a driving method of setting voltage changing times during the first and second voltage changing processes 54 and 56 as integral multiples of the resonance frequency Tc of the pressure wave to prevent the pressure wave from reverberating after the ejection of ink droplets, thereby preventing the occurrence of satellites.
Results of experiments, however, show that even the meniscus controlling (pull and push) driving method (FIG. 17) described in the above publication can reduce the ink droplet size to only about 90% of the nozzle diameter, and it is thus practically difficult to obtain fine ink droplets of 20 xcexcm or less to achieve high quality recording. That is, results of ejection experiments conducted by the inventors with a nozzle diameter of 30 xcexcm, a pressure wave resonance frequency Tc of 14 xcexcs, and a droplet speed Vd of 6 m/s and using the driving voltage waveform shown in FIG. 17 show that the droplet size obtained (equivalent size calculated from the total amount of ejected ink including satellites) has a lower limit of 28 xcexcm even if the values of the reference voltage V1, the voltage changing time (falling time) t1 during the first voltage changing process 54, the voltage maintaining time t1xe2x80x2 during the voltage maintaining process 55, and the voltage changing time (rising time) t2 during the second voltage changing process 56 are varied and combined.
Further, if fast driving is executed with the inversely trapezoidal voltage waveform shown in FIG. 17, the pressure wave reverberates significantly after the ink ejection, resulting in unstable ejection such as delayed satellites or inappropriate ejection. In the experiments conducted by the inventors, when driving frequency exceeded 8 kHz, bubbles were entrained to the interior of the nozzle or satellite droplets adhered to peripheries of the nozzle, so that a decrease in droplet speed Vd and inappropriate ejection were observed. It has been assured that the head used in the experiments can be driven at 10 kHz or more with the trapezoidal driving voltage waveform shown in FIG. 16, so that the inappropriate ejection evidently arises from a reverberated pressure wave, which is caused by the inversely trapezoidal driving voltage waveform.
On the other hand, in the driving voltage waveform shown in FIG. 17, if the falling time t1 and the rising time t2 are set equal to integral multiples of the resonance frequency Tc, the ejection can be kept stable but it becomes difficult to obtain fine droplets, as described in Japanese Patent Laid-Open No. HEI 2-192947. That is, the results of the experiments conducted by the inventors indicate that if the rising/falling time (t1/t2) is made equal to the resonance frequency Tc, the fine droplets obtained have a size of 35 xcexcm when the nozzle diameter is 30 xcexcm. Thus, it is difficult to obtain a droplet size equal to or smaller than the nozzle diameter.
The present invention is provided in view of the above described circumstances, and it is an object of the present invention to provide a method for driving an ink jet recording head which method enables fine ink droplets having a smaller size (for example, about 20 xcexcm) than a nozzle to be stably ejected even at a high frequency.
To attain the above object, the invention set forth in claim 1 provides a method for driving an ink jet recording head which method applies a driving voltage to an electro-mechanical converter to deform the electromechanical converter to thereby change a pressure in the pressure generating chamber filled with an ink, thus ejecting ink droplets through a nozzle in communication with the pressure generating chamber, the method being characterized in that a voltage waveform of the driving voltage comprises at least a first voltage changing process for applying a voltage in a direction that increases a volume of the pressure generating chamber, a second voltage changing process for then applying a voltage in a direction that reduces the volume of the pressure generating chamber, a third voltage changing process for applying a voltage in a direction that increases the volume of the pressure generating chamber again, and voltage changing times t2 and t3 during the second and third voltage changing processes are set to have such lengths as shown below, relative to a resonance frequency Tc of a pressure wave generated in the pressure generating chamber:
0 less than t2 less than Tc/2
0 less than t3 less than Tc/2.
The invention set forth in claim 2 is the method for driving an ink jet recording head according to 1, characterized in that a start time of the third voltage changing process is the same as an end time of the second voltage changing process.
The invention set forth in claim 3 is the method for driving an ink jet recording head according to claim 1 or 2, characterized in that the voltage waveform of the driving voltage includes a fourth voltage changing process for applying a voltage in a direction that reduces the voltage of the pressure generating chamber, after the first voltage changing process, the second voltage changing process, and the third voltage changing process.
The invention set forth in claim 4 is the method for driving an ink jet recording head according to claim 3, characterized in that a voltage changing time t4 during the fourth voltage changing process is set as follows relative to the resonance frequency Tc of the pressure wave generated in the pressure generating chamber:
0 less than t4 less than Tc/2.
The invention set forth in claim 5 is the method for driving an ink jet recording head according to claim 3 or 4, characterized in that a time interval between a start time of the second voltage changing process and a start time of the fourth voltage changing process is set substantially half the length of the resonance frequency Tc of the pressure wave generated in the pressure generating chamber.
The invention set forth in claim 6 is the method for driving an ink jet recording head according to any of claims 1 to 5, characterized in that the electomechanical converter is a piezoelectric actuator.
The invention set forth in claim 7 is the method for driving an ink jet recording head according to any of claims 1 to 5, characterized in that an ink jet recording head with the nozzle of 20 to 40 xcexcm opening diameter is driven to eject ink droplets of 5 to 25 xcexcm size.
A theoretical ground for the validity of the present invention will be explained with reference to a lumped-parameter equivalent circuit model.
FIG. 12(a) is an equivalent electrical circuit diagram showing that the ink jet recording head shown in FIG. 1 is filled with an ink. In FIG. 12(a), reference m0 denotes the inertance (acoustic mass) [kg/m4] of a vibration system comprising a piezoelectric actuator 4 and a diaphragm 3, reference m2 denotes the inertance of an ink supply hole 6, reference m3 denotes the inertance of a nozzle 7, reference r2 denotes an acoustic resistance [Ns/m5] from the ink supply hole 6, reference r3 denotes an acoustic resistance from the nozzle 7, reference c0 denotes the acoustic capacity [m5/N] of the vibration system, reference c1 denotes the acoustic capacity of the pressure generating chamber 2, reference c2 denotes the acoustic capacity of the ink supply hole 6, reference c3 denotes the acoustic capacity of the nozzle 7, and reference xcfx86 denotes a pressure [Pa] effected on the ink.
In this case, if the piezoelectric actuator 4 comprises a rigid laminated piezoelectric actuator, the inertance m0 and acoustic capacity C0 of the vibration system are negligible. Accordingly, the equivalent circuit in FIG. 12(a) is approximately represented by the equivalent circuit in FIG. 12(b).
Additionally, if it is assumed that the relation expression m2=km3 is established between the inertances m2 and m3 of the ink supply hole 6 and the nozzle 7 and that the relation expression r2=kr3 is established between the acoustic resistances r2 and r3 from the ink supply hole 6 and the nozzle 7 and if circuit analysis is carried out for a case where a driving voltage waveform having a rising angle xcex8 is input as shown in FIG. 13(a), then a volume velocity u3xe2x80x2 [m3/s] in the nozzle section 7 during a rising time 0xe2x89xa6txe2x89xa6t1 is given by Equation (2).                                           u            3            xe2x80x2                    ⁡                      (                          t              ,              θ                        )                          =                                                                              c                  1                                ⁢                tan                ⁢                                  xe2x80x83                                ⁢                θ                                            (                                  1                  +                                      1                    k                                                  )                                      ⁡                          [                              1                -                                                      w                                          E                      c                                                        ⁢                                      exp                    ⁡                                          (                                                                        -                                                      D                            c                                                                          ·                        t                                            )                                                        ⁢                                      sin                    ⁡                                          (                                                                                                    E                            c                                                    ⁢                          t                                                -                                                  φ                          0                                                                    )                                                                                  ]                                ⁢                      (                          0              ≤              t              ≤                              t                1                                      )                                              (        2        )            
Here is,             E      c        =                                        1            +                          1              k                                                          c              1                        ⁢                          m              3                                      -                  D          c          2                                D      c        =                  r        3                    2        ⁢                  xe2x80x83                ⁢                  m          3                                w      2        =                  1        +                  1          k                                      c          1                ⁢                  m          3                                φ      0        =                  tan                  -          1                    ⁢                        E          c                          D                      c            ⁢                          xe2x80x83                                          
Next, the volume velocity obtained using a driving voltage waveform of a complicated shape (trapezoid) as shown in FIG. 13(b) can be determined by superposing together pressure waves generated at nodes (points A, B, C, and D) of the driving voltage waveform. That is, the volume velocity u3 [m3/s] in the nozzle section 7 as occurring in the driving voltage waveform in FIG. 13(b) is given by Equation (3).                                                                                                               u                    3                                    ⁡                                      (                    t                    )                                                  =                                                      u                    3                    xe2x80x2                                    ⁡                                      (                                          t                      ,                                              θ                        1                                                              )                                                                                                      (                                  0                  ≤                  t                  ≤                                      t                    1                                                  )                                                                                                                              u                    3                                    ⁡                                      (                    t                    )                                                  =                                                                            u                      3                      xe2x80x2                                        ⁡                                          (                                              t                        ,                                                  θ                          1                                                                    )                                                        +                                                            u                      3                      xe2x80x2                                        ⁡                                          (                                                                        t                          -                                                      t                            1                                                                          ,                                                  θ                          2                                                                    )                                                                                                                          (                                                      t                    1                                    ≤                  t                  ≤                                                            t                      1                                        +                                          t                      1                      xe2x80x2                                                                      )                                                                                                                                                                                                      u                          3                                                ⁡                                                  (                          t                          )                                                                    =                                            ⁢                                                                                                    u                            3                            xe2x80x2                                                    ⁡                                                      (                                                          t                              ,                                                              θ                                1                                                                                      )                                                                          +                                                                              u                            3                            xe2x80x2                                                    ⁡                                                      (                                                                                          t                                -                                                                  t                                  1                                                                                            ,                                                              θ                                2                                                                                      )                                                                          +                                                                                                                                                                              ⁢                                                                        u                          3                          xe2x80x2                                                ⁡                                                  (                                                                                    t                              -                                                              (                                                                                                      t                                    1                                                                    +                                                                      t                                    1                                    xe2x80x2                                                                                                  )                                                                                      ,                                                          θ                              3                                                                                )                                                                                                                                                                        (                                                                            t                      1                                        +                                          t                      1                      xe2x80x2                                                        ≤                  t                  ≤                                                            t                      1                      xe2x80x2                                        +                                          t                      2                                                                      )                                                                                                                                                                                                      u                          3                                                ⁡                                                  (                          t                          )                                                                    =                                            ⁢                                                                                                    u                            3                            xe2x80x2                                                    ⁡                                                      (                                                          t                              ,                                                              θ                                1                                                                                      )                                                                          +                                                                              u                            3                            xe2x80x2                                                    ⁡                                                      (                                                                                          t                                -                                                                  t                                  1                                                                                            ,                                                              θ                                2                                                                                      )                                                                          +                                                                                                                                                                              ⁢                                                                                                    u                            3                            xe2x80x2                                                    ⁡                                                      (                                                                                          t                                -                                                                  (                                                                                                            t                                      1                                                                        +                                                                          t                                      1                                      xe2x80x2                                                                                                        )                                                                                            ,                                                              θ                                3                                                                                      )                                                                          +                                                                                                                                                                              ⁢                                                                        u                          3                          xe2x80x2                                                ⁡                                                  (                                                                                    t                              -                                                              (                                                                                                      t                                    1                                                                    +                                                                      t                                    1                                    xe2x80x2                                                                    +                                                                      t                                    2                                                                                                  )                                                                                      ,                                                          θ                              4                                                                                )                                                                                                                                                                        (                                  t                  ≥                                                            t                      1                                        +                                          t                      1                      xe2x80x2                                        +                                          t                      2                                                                      )                                                    }                            (        3        )            
When the volume velocity u3 is actually determined for the driving voltage waveform in FIG. 13(a) using Equation (3), the result indicates that temporal variations in volume velocity u3 vary significantly depending on the rising time t1. FIG. 14 shows an example. In an area corresponding to t1 less than Tc (Tc: resonance frequency of pressure waves), the volume velocity u3 becomes zero earlier (the time (txe2x80x3)) as the rising time t1 decreases (a)xe2x86x92(b)xe2x86x92(c) in FIG. 14.
The particle velocity in the figure is defined as a value obtained by dividing the volume velocity u3xe2x80x2 of the nozzle section 7 by the opening area of the nozzle. Thus, since the driving voltage waveform significantly varies the waveform of the volume velocity of the nozzle section 7, this can be used as a principle of fine-droplet ejection. This is because the volume q of ejected droplets is substantially proportional to the shaded area in FIG. 14, as is apparent from what is expressed by Equation (4).
qxe2x88x9d∫txe2x80x2txe2x80x3u(t)dtxe2x80x83xe2x80x83(4)
That is, setting a small rising time t1 reduces the area of the shaded portion, thereby obtaining a small volume of droplets (droplet size) q. In particular, fine droplets can be ejected by setting the rising time t1 equal to or shorter than half of the resonance frequency Tc of the pressure wave (this also applies to the falling time t2).
If the driving voltage waveform shown in FIG. 17 is used to execute meniscus control (pull and push), it is particularly desirable for fine-droplet ejection to set the rising time t2 equal to or shorter than half of the resonance frequency Tc of the pressure wave. This is because ink droplets can be made further smaller due to the droplet size reducing effect based on the conventional meniscus control as well as the above-described variation of the volume velocity waveform (a decrease in shaded area).
However, it is very difficult to obtain fine droplets of 20 xcexcm size simply by setting a shorter rising time t2 for the inversely trapezoidal driving voltage waveform shown in FIG. 17. Thus, if the piezoelectric actuator 4 is imparted with a third voltage changing process (voltage lowering process) for rapidly increasing the volume of the pressure generating chamber 2 immediately after the driving voltage waveform has risen, as shown in FIG. 4(a), then the shaded area further decreases to enable the ink droplets to be made further smaller, as shown in FIG. 5(a). Additionally, the effect of the falling edge on the reduction of the droplet size depends on the time interval between the rising and falling edges; if the falling edge is set to appear immediately after the rising edge, that is, the start time of the third voltage changing process is set equal to the end time of the second voltage changing process, as shown in FIG. 4(b), the smallest droplet diameter is obtained as shown in FIG. 5(b).
Further, as described above, the use of a driving voltage waveform having a rapid rising or falling edge causes the pressure wave to reverberate significantly after the ejection, so that a problem such as generation of satellites or a reduced stability of fast driving is likely to occur. Thus, according to the inventions set forth in claims 3, 4, and 5, a fourth voltage changing process (voltage raising process) for generating pressure waves to restrain reverberation is provided after the third voltage changing process. This serves to compensate for previously generated pressure waves to prevent reverberation, while improving the ejection stability.